1. Field of the Invention
The present invention relates to a process for the microheterogeneously catalyzed preparation of alkali metal alkoxides, in particular potassium tert-butoxide, from alkali metal amalgams and alcohols.
2. Background of the Invention
Alkali metal alkoxides, particularly of alcohols having up to 4 carbon atoms, are valuable chemicals. They are used, for example, as catalysts in the synthesis of many organic compounds. Here, chiefly the alkoxides of sodium and of potassium have achieved practical importance. A number of methods are known for preparing alkali metal alkoxides (see, F. A. Dickes, Ber. Dtsch. Chem. Ges. 63, 2753 [1930]). Solutions of alkali metal hydroxides in an alcohol contain alkali metal alkoxide in equilibrium. Removing the water present in this equilibrium, for example by distillation, gives pure alkali metal alkoxides. However, particularly in the case of low-boiling alcohols, this way of shifting the equilibrium requires a great deal of energy.
Pure alkali metal alkoxides are obtained directly by dissolving an alkali metal in the corresponding alcohol. Sodium and potassium react violently with lower aliphatic alcohols such as methanol and ethanol, evolving hydrogen. Higher alcohols such as propanols and butanols are preferably reacted with the alkali metals at above the melting point of the latter, if desired under pressure and with stirring.
However, alkali metal is an expensive starting material. It is more economical to use the liquid alkali metal amalgam obtained in chlor-alkali electrolysis by the mercury process as source of alkali metal.
The reaction of alkali metal amalgam with alcohols to give alkoxides and also the use of catalysts for this reaction are known.
The process disclosed in EP-A 0 177 768 enables the reaction between the amalgam and the alcohol to be carried out quite quickly. The reaction is carried out using a bed of granular anthracite whose surface is coated with heavy metal oxide or a mixture of heavy metal oxides. The amalgam and alcohol are fed in continuously in countercurrent and the products are taken off continuously.
These processes are, however, quite unsatisfactory. For example, only from 60 to 80% of the sodium introduced by means of the amalgam are reacted with methanol. A conversion of up to 100% in the preparation of MeONa is achieved only if the method of DE 196 21 466 is followed.
However, even now, the preparation of, for example, potassium tert-butoxide is only possible in an economically still unsatisfactory space-time yield, even when the reaction is carried out in an autoclave at 140.degree. C. and 7 bar.